Advice needed on 1 month old chicks!

We have 3 almost three month old chicks, and 9 one month old chicks. We also have a really large & secure henhouse & run (which the 3 three month olds are in currently, the one months are in a small, temporary place).

My question is: could we put our one months in the same run & house as our 3 months, or is that a good way for them to get hurt or "pecked" more so than normal?

With that age difference, I wouldn't put them together until the younger ones are at least 16 weeks old.

Older birds can be MEAN... I just put four 16 week olds in with some that are 25 weeks old... and the 16 week old birds spend most of the time on the roosts because they are terrified of the older birds.

Even at the same age putting new birds in with an existing flock doesn't always wind up like the Brady bunch. I have 4 birds now that spend all day inside on the roosts because the big bullies are out in the run. The only way they come down is when the doors are open and the bullies go to another part of the yard. I really hope they get over this soon, I feel bad for the new girls.